[Canberrauav] CMAC Helicopter Testing and Airframe Issue.

Greg Oakes captain351 at gmail.com
Tue Feb 16 08:54:51 AEDT 2016


Chris,

Thank you for the information on the carbon fibre.  Looking into this
further the issue appears to be the epoxy used in the current carbon frames
combined with the operating temperature of the petrol engine.

I have managed to make up some aluminum inserts for the frames and will use
a larger surface area metal plate to dissipate some of the heat around the
mount point plus distribute the load.

I looked at a few epoxies yesterday from various suppliers and even the
weld types have temperature limits in the operating temperature range of
the engine so will soften. I was able to get a new version of medium thread
lock though that has a higher effective anaerobic cure range up to 180C so
this should work at the 135C temp of the engine.  At least worth a try and
its still low enough I will be able to get the bolts out later on.

Good Idea about cutting new frames.  I actually have a new set same as the
ones I have now so could look to the future using them as template for a
better grade or alternate material.  The current Trex 700N has seen a lot
of flying so is reaching its end of service life.  I suspect we only need
to run this a for short while before switching to our new helis.

Greg

On Mon, Feb 15, 2016 at 2:02 PM, <christopher.d.gough at gmail.com> wrote:

> Hi Greg,
>
> The carbon itself is unlikely to be effected at those temps (carbon cloth
> won't cut with a co2 laser), but some epoxies definitely go gooey at low
> temps. High-temp automotive epoxies (e.g. JB Weld) have additives to
> increase conductivity (e.g. powdered iron) as well as a higher glass
> transition temperatures. The additives improve adhesion with metal
> surfaces, so you might consider attaching the washers permanently.
>
> I don't know anything about aerospace-grade high temp epoxies, but JB Weld
> and equivalents are available at automotive stores.
>
> Another idea, is there a way we could scan these plates and get new ones
> cut with a water jet (either high-temp epoxy/cf or even the right kind of
> aluminium).
>
> Chris
> ..
>
> > On 15 Feb 2016, at 11:09 AM, Greg Oakes <captain351 at gmail.com> wrote:
> >
> > Folks,
> >
> > On the weekend at CMAC we had successful test flying of the Trex 700
> fitted with the combination of pxihawk and using an external 3-axis gyro
> stabilization system.  The idea behind this setup is to provide an
> independent stabilization system for emergency bail out, and to counter
> some of the additional vibration effects with larger capacity petrol
> helicopters.  With the current setup all control channels are directly
> passed through to the external 3 axis MEMS Gyro from the pixhawk SBUS out
> in Acro mode. With this separation we can apply more efficient vibration
> isolation of the Pixhawk which would otherwise compromise the function of
> the Gyro's which need tighter coupling to the airframe.
> >
> > Whilst we have more testing to do, we flew the Trex 700 in both
> Stabilise and ALT hold with no issue other than some minor tuning needed to
> correct the demanded versus achieved pitch, roll and yaw values and a
> little Z-axis oscillation in ALT hold altitude change.
> >
> > On a different aspect we have encountered an issue with the current
> Trex700N engine mounting coming loose due to excessive wear on the actual
> carbon fiber frame where the 3mm countersunk bolts, which precisely align
> the engine mount bearers, have worked themselves into the carbon fabric
> matrix.  Whilst the helicopter is old and done many hours I am wondering if
> anyone has seen or encountered evidence the additional heat from a petrol
> engine, could break down the carbon fiber faster.  The trex 700N was
> originally designed to take a Nitro glow engine which runs cooler.  It is
> either mechanical vibration over time under the bolt stress has broken down
> the carbon fiber sealed surface on same stress but addition heat contact.
> Looking for opinion advice on the likely suspect here?
> >
> > The issue we have now is trying to stop the problem spreading further
> and also repairing if possible the top surface of the carbon fiber around
> the engine mount holes.  What I have done so far is to place a larger
> diameter and thicker aluminum washer to distribute the load and try an
> dissipate the heat transfer from the motor to the alloy motor mounts.  Also
> because the frame engine mount holes are larger dia than the mount bolts, I
> needed to insert a spacer to maintain the correct engine alignment.  Using
> a slightly larger bolt is problematic due to how close the engine mount
> holes are to the edge of the CF frames.
> >
> > If you have any details of heat effects on breaking down carbon fiber
> bonding this would be appreciated.  Also are there any epoxies / glues
> which you recommend , that have high heat application?(wont soften at the
> temperatures transferred from the petrol engine crankcase.  I need to bond
> the aluminum spacers to seal the CF surface.  Fortunately during earlier OS
> GT15Hz we measured the temperature at 130-140 C around the cylinder hear
> and base of the spark plug.  The max temp we could go to before exceeding
> the manufacturers engine spec is 160-170C. So epoxy would need to survive
> at the fore mentioned temperatures.
> >
> > Eventually we will rebuild the heli and look at other materials and
> newer composites.  But for now need to extend the operational hours (within
> safety) if possible.
> >
> > Greg
> >
> >
> >
> >
>
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